docs: finalize 1.x to 2.0 migration guide

- Rename to migration-1.x-to-2.0.md; name the release 2.0, not 2.x
- Add a Why upgrade to 2.0 section listing only 2.0 additions
- Rewrite in active voice; remove filler and passive constructions
- Rename batching heading to Removed 1.x-only names
- Add max_concurrency in-flight semantics section
- Add replay operation identity validation (#698)
- Add wait_for_condition unreadable-polling-state failure
- Scope RetryableSerDesError guidance to invocation replay and
  step semantics; scope exhaustion to create_wait_strategy
- Correct callbacks note: create_callback().result() still exists
- Move the experimental plugin note to its own linked section
- Mark the grep checklist as non-exhaustive

Author: yaythomas